The present invention relates generally to fiber optical couplers, and more particularly to the fabrication of polarization-preserving fiber optical couplers.
Optical fiber-to-fiber couplers for coupling an optical beam from one or more fibers to another fiber or fibers are known in the art.
Single mode fiber couplers have been fabricated in several ways with nominally isotropic fiber. One of the difficulties with these couplers has been their apparent inability, in most cases, to preserve input polarization states through the coupler. Instead, linearly polarized inputs are often transformed into elliptical states on exiting the coupler. The recent development of high-birefringence fibers--called polarization-preserving single mode (PPSM) fibers--that can maintain a given state of polarization over long lengths, raised the possibility of using them to fabricate couplers which maintain polarization throughout the coupler. Since alignment of the polarization axes of the fibers forming the coupler was thought to be crucial, and this latter task is technically difficult, only recently has such a coupler been reported. The term "polarization axes" is used herein in its conventional sense as the principal directions in a birefringent fiber for which the velocity of the linearly-polarized wave is different. This prior art coupler is disclosed in Electronic Letters 18, pp. 9624 (1981) by M. Kawachi et al. The method of fabricating this coupler disadvantageously focuses on a scheme to achieve mutual alignment of the fiber polarization axes.